c++设计模式-策略模式

策略模式

策略模式

适用情况

假如你要开发一个地图导航应用，一开始你只能规划汽车线路，但是后面你要继续开发人行线路、飞机线路等，这些线路算法并不通用，需要重写某段算法，这时候你可以把重写放到一个新的类，并把这些类作为一个参数

优缺点

优点：可以运行时切换、算法实现和算法使用隔离、使用组合来代替继承、 开闭原则
缺点：如果算法改动不大，新建类会显得复杂，而且使用多函数可能会更方便

与其他模式关系

这个等学完在对比

代码实例

class Strategy
{
public:
    virtual ~Strategy() {}
    virtual std::string DoAlgorithm(const std::vector<std::string> &data) const = 0;
};

class Context
{
private:
    Strategy *strategy_;

public:
    Context(Strategy *strategy = nullptr) : strategy_(strategy)
    {
    }
    ~Context()
    {
        delete this->strategy_;
    }

    void set_strategy(Strategy *strategy)
    {
        delete this->strategy_;
        this->strategy_ = strategy;
    }

    void DoSomeBusinessLogic() const
    {
        // ...
        std::cout << "Context: Sorting data using the strategy (not sure how it'll do it)\n";
        std::string result = this->strategy_->DoAlgorithm(std::vector<std::string>{"a", "e", "c", "b", "d"});
        std::cout << result << "\n";
        // ...
    }
};

class ConcreteStrategyA : public Strategy
{
public:
    std::string DoAlgorithm(const std::vector<std::string> &data) const override
    {
        std::string result;
        std::for_each(std::begin(data), std::end(data), [&result](const std::string &letter) {
            result += letter;
        });
        std::sort(std::begin(result), std::end(result));

        return result;
    }
};
class ConcreteStrategyB : public Strategy
{
    std::string DoAlgorithm(const std::vector<std::string> &data) const override
    {
        std::string result;
        std::for_each(std::begin(data), std::end(data), [&result](const std::string &letter) {
            result += letter;
        });
        std::sort(std::begin(result), std::end(result));
        for (int i = 0; i < result.size() / 2; i++)
        {
            std::swap(result[i], result[result.size() - i - 1]);
        }

        return result;
    }
};

void ClientCode()
{
    Context *context = new Context(new ConcreteStrategyA);
    std::cout << "Client: Strategy is set to normal sorting.\n";
    context->DoSomeBusinessLogic();
    std::cout << "\n";
    std::cout << "Client: Strategy is set to reverse sorting.\n";
    context->set_strategy(new ConcreteStrategyB);
    context->DoSomeBusinessLogic();
    delete context;
}

int main()
{
    ClientCode();
    return 0;
}